I. Field of the Invention
Many commercially important coal measures contain substantial proportions of ash. Almost all uses of coal can be accomplished more efficiently by reducing the moisture and ash content of the coal. Especially where the coal is used for generation of heat or electrical power, the moisture content should be reduced to the lowest practical level, and the higher heating value should be as high as possible. Reductions in moisture and ash content produce corresponding increases in the higher heating value of the coal.
My invention relates to a novel chemical coal conditioning process which uses complexing agents such as sodium gluconate in alkaline solution to form soluble compounds with cations such as Al.sup.+3, Fe.sup.+3, Mg.sup.+2, and Ca.sup.+2. I have found that the formation of such compounds reduces the adsorption and/or desorbs clay slimes from the coal surface. In a preferred embodiment of my invention, deashing and dewatering is further enhanced by addition of surfactants such as those disclosed in my co-pending application, Ser. No. 8-047787 filed Apr. 15, 1993, the disclosure of which is incorporated herein by reference.
II. Description of the Prior Art
Some information is available concerning the chemistry of clay/coal interaction. In Burdon, R. G.; Booth, R. W. and Mishra, S. K., "Factors Influencing the Selection of Process for the Benificiation of Fine Coal", Proceedings of the 7th International Coal Preparation Congress, Sydney, N.S.W., Australia, Paper E.1 (1976), for example, a theoretical discussion of the mechanisms of clay adsorption on the organic constituents of coal is presented, and it is hypothesized that Fe.sup.+3 and Al.sup.+3 cations play a role in promoting such adsorption. Eamer, B. J., "Surface Chemical Treatment of Fine Coal With Slime Problem", M. P. Appl. Sc. Thesis, Western Australian Institute of Technology (1981) and Mishra, S. K. and Eamer, B. T., "Effect of Clay Slimes On Flotation Behavior of Coal", Paper presented at the Fine Particle Society Conference (1985) include similar discussions.
Conventional dewatering and deashing processes may use a rotary drum vacuum filter or comparable device to remove ash and water from slurries of fine coal. A variety of chemical pretreatments have been disclosed to improve the performance of these process steps. U.S. Pat. No. 5,192,338 (Waugh), for example, discloses a treating process using serial treatment with aqueous solutions of citric acid and glycerol at elevated temperature for a period of 30 to 45 minutes to beneficiate coal. Mention is made of the use of "organic complexing agents capable of complexing with metal cations" (col. 3, lines 38-39 and 51-56), but the only reagents disclosed in that category are ethylene diamine tetraacetic acid and its disodium salt; 8-hydroxyquinoline and mercaptoethanol.
U.S. Pat. No. 5,089,142 discloses using sodium hexametaphosphate to control slime formation in centrifugal dewatering processes.
Some investigators, as in U.S. Pat. No. 4,231,868 (Wang, et. al.), have suggested adding sulfosuccinate surfactants to slurries of fine coal in order to improve dewatering efficiency. A similar approach was suggested in U.S. Pat. No. 4,985,162 (Cole). Other researchers also have suggested using sulfur and nitrogen-based compounds, as in U.S. Pat. No. 4,897,201 (Yamomoto, et. al.). The introduction of sulfur-containing surfactants into the coal/water slurry, however, requires application rates substantially higher than those used in my invention. Such surfactants also cause undesirable foam formation downstream of the filtration operation that interferes with subsequent coal processing. Therefore, additional anti-foaming agents may be required. The addition of sulfur to the processed coal is also undesirable from an air pollution standpoint.
Still other investigators have suggested the addition of other reagents to the coal/water slurry. A quaternary amine surfactant was suggested in U.S. Pat. No. 4,892,663 (Keys). U.S. Pat. No. 4,290,897 discloses the addition of organopolysiloxanes and U.S. Pat. No. 4,447,344 (Roe) discloses a variety of ethoxylated alcohols.
None of these references, alone or in combination, suggests that the effectiveness of the coal cleaning processes can be improved by using any complexing agents in basic solution, or by following such treatment with the addition of surfactants such as sodium laureth-13 carboxylate salt in aqueous solution in the form of a foam.